Because we don't have gills is the short answer, Maggie. But we've got some space we have to fill, so here goes a more scientific explanation:

According to the HowStuffWorks website, whose makers have far bigger brains than ours here at SYA Central, our lungs don't have enough surface area to absorb enough oxygen from water to survive. The oxygen that makes up liquid water is bound to two hydrogen atoms (H20), making it useless to our lungs.

Lungs are pretty cool organs, when you think about it. They do their job — exchanging carbon dioxide, a gas your body needs to get rid of, for oxygen, a gas your body needs — an average of 12 to 16 times per minute without you even thinking about it.

Fish extract oxygen from the water by using their gills. And though there's far less oxygen in water than in air, fish don't need as much because they're cold-blooded.

Whales, on the other hand, are warm-blooded, so they've got to breathe air like humans do.

Why do organisms from fish to humans need oxygen in the first place? To help convert food into energy in our cells. That process creates the waste gas carbon dioxide, which our lungs help us get rid of.

There have been experiments with humans breathing other liquids, such as fluorocarbons, but we'll leave that for another exciting episode of Since You Asked.

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